No, the "people repeating this version of the story" are not just a handful of western propagandists and nationalists. This story is accepted by everyone except for a few MLs.

It's accepted by anyone with a bone to pick with the soviet union or AES states in general. If you look for information online on this it almost always comes from liberal sources. It's also "generally accepted" that China is committing genocide against the Uyghurs, and it's mostly MLs who believe otherwise. It doesn't stop it from being bunk.

On April 14, 1943 he wrote "We are now using the discovery of 12,000 Polish officers, murdered by the GPU, for anti-Bolshevik propaganda on a grand scale.

And on 8 May 1943, he wrote:

“Unfortunately, German ammunition has been found in the graves at Katyn It is essential that this incident remains top secret. If it were to come to the knowledge of the enemy the whole Katyn affair would have to be dropped.”

Evidence of Soviet guilt was provided in the 90s - Beria's letter to Stalin comes to mind.

A letter which is of dubious authenticity. Why didn't Khrushchev use such a letter in his smear campaign against Stalin, if it was genuine? Why keep it a secret for so long after the Soviet State became hostile to the figure of Stalin? Viktor Iliukhin also produced evidence for the drafts of the forgery in 2010, and the contents contradict what we do know about the site anyway. You could also argue that he made a forgery out of his own forgery documents, and we would be arguing over who forged which documents endlessly. But we know that Khrushchev lied about Stalin repeatedly, Gorbachev wouldn't be beneath it either.

Recent investigations into the mass graves done by polish archeologists showed that the victims didn't have anything made after 1940 on them.

"Recent investigations" until about 2012 also pointed towards a 1941 date:

https://web.archive.org/web/20131219001802/http://www.kresykedzierzynkozle.home.pl/attachments/File/Rap.pdf

It actually says that it couldn't have been done earlier than 1941:

Z pewnością stwierdzono, że zbrodnia została dokonana nie wcześniej niż w 1941 roku. (p.4)

It is confirmed with certainty that the crime did not take place earlier than 1941.

Transcript of the article:

The story of the mass graves at Katyn, which caused a world sensation two years ago, was a propaganda stunt staged by Goebbels and Ribbentrop to cause a split between Russia and her western allies, says a report received here through special channels that is supported by a message from Oslo tonight. A Himmler close collaborator, SS Brigade Leader Schellenberg, is declared to have given this sensational information during an examination at Allied Headquarters in Germany last Tuesday. He is quoted as saying that 12,000 bodies were taken from German concentration camps and attired in old Polish uniforms to make them appear to be Polish officers.

Tonight a corroborative report was received from Oslo, where Erik Johansen–recently repatriated prisoner from the Sachsenhausen concentration camp in Germany–tells an interesting story about German production of false identification documents for bodies in Katyn mass graves.

Johansen says a special section of the concentration camp was completely isolated and strongly guarded by SS men, whereupon forty to sixty Jewish prisoners were picked out to forge the documents. They received the best optical instruments obtainable so the work could be done to perfection. They made passports, letters, etc. and even wallets, which were treated with a special chemical fluid to make them look worn.

Before the German capitulation all machines, instruments and material used were destroyed and the Jewish specialists were killed to prevent the secret from getting out, he said.